In a sealed battery such as a lithium ion secondary battery to be mounted in a vehicle and other devices, gas may be stored due to overcharging, excessive rise in temperature, breakage by external forces, and others. Furthermore, the gas may cause a rise in internal pressure of the battery. A battery case of the sealed battery is therefore usually formed with a breakable portion for safety. For instance, some batteries include a battery case partially formed with a member deformable by battery internal pressure so that the member is deformed to break a breakable portion. In those batteries, the breakage of the breakable portion cuts off a current path or provides a gas outlet between the inside and the outside of a secondary battery, and so on.
For example, Patent Literature 1 discloses a secondary battery provided with a diaphragm attached to an electrode terminal on the inner side of the battery. In this device of the Literature, a part of a breakable portion is fixed to the diaphragm. Accordingly, when the diaphragm is deformed by a rise in internal pressure, the breakable portion is broken or cleaved. A current path at that place in the battery is thus cut off.